docs: def get_pdf(article, debug=False):
      """
      Download an article PDF from arXiv.
  
      :param article:
          The ADS article to retrieve.
  
      :type article:
          :class:`ads.search.Article`
  
      :returns:
          The binary content of the requested PDF.
      """
  
      print('Retrieving {0}'.format(article))
  
      identifier = [_ for _ in article.identifier if 'arXiv' in _]
      if identifier:
          url = 'http://arXiv.org/pdf/{0}.{1}'.format(identifier[0][9:13],
              ''.join(_ for _ in identifier[0][14:] if _.isdigit()))
      else:
          # No arXiv version. Ask ADS to redirect us to the journal article.
          params = {
              'bibcode': article.bibcode,
              'link_type': 'ARTICLE',
              'db_key': 'AST'
          }
          url = requests.get('http://adsabs.harvard.edu/cgi-bin/nph-data_query', 
              params=params).url
  
      q = requests.get(url)
      if not q.ok:
          print('Error retrieving {0}: {1} for {2}'.format(
              article, q.status_code, url))
          if debug: q.raise_for_status()
          else: return None
  
      # Check if the journal has given back forbidden HTML.
      if q.content.endswith('</html>'):
          print('Error retrieving {0}: 200 (access denied?) for {1}'.format(
              article, url))
          return None
      return q.content